I have found a lot of these seeds (?) in my yard over the years.  It is hard. Sometimes they appear to be buried and other times just in various parts of the yard.  I have a 300+ year old oak tree in my yard. Many little creatures come through: squirrels, chipmunks, red foxes, deer, ground hogs, raccoons, along with lots of birds.

Just recently I found a crepe myrtle growing in my garden that I did not plant.

Thank you for your time to help me with this question as to what this seed might be.

Gigi